A third successive victory for the Black Cats - who had beaten Southampton and Manchester City in their previous two matches - appeared to be on the cards when John O'Shea put them ahead in the 40th minute.
But Spurs hit back with quickfire goals shortly after the break, Carlos Cuellar heading into his own net before Aaron Lennon struck.
O'Neill said: "It was a tough game and we knew it would be. We fought so hard to get in front and then gave it away in a two-minute spell just after half-time.
"We fought valiantly throughout the game I felt. There's been a big effort in the last couple of games and it has taken a lot out of the players.
"They still keep going right to the very end. We lack in certain things but in heart and soul we certainly don't. It is very seldom in terms of commitment that I would be unhappy with the team.
"I'm not so sure Tottenham created so many chances - I don't remember them causing us too many problems in the first half.
"In the second half they got the two goals in quick succession. They are a very fine side. That is why they are challenging for the Champions League."